1. Technical Field of the Invention
The present invention relates to a dual mode communications system employing spread spectrum multiple access and frequency division multiple access techniques and, in particular, to the handoff of a mobile station between the spread spectrum portion of the system and the frequency division portion of the system.
2. Description of Related Art
Multiple access communications systems are well known in the art. The primary advantage of such communications systems is that many mobile stations may simultaneously engage in radio frequency communications while sharing common radio frequency spectrum resources. Two well known types of multiple access communications systems are narrowband (analog and/or digital) frequency division multiple access (FDMA) communications systems and wideband (digital) spread spectrum or code division multiple access (CDMA) communications systems.
Many kinds of frequency division multiple access communications systems are known in the art including: the analog Advanced Mobile Phone Service (AMPS); the digital Advanced Mobile Phone Service (D-AMPS); and the digital Global System for Mobile (GSM) communications. In a frequency division multiple access communications system, each cell operates with an assigned set of transmission frequencies. In an analog (i.e., non time division multiplexed) FDMA communications system, any given frequency comprises a channel. In a digital, time division multiplexed TDMA/FDMA communications system, however, each frequency is divided into a plurality of time slots which comprise the channels. The set of channels assigned to each cell includes at least one control channel (CCH) operable using FDMA or FDMA/TDMA multiplexing in digital mode, and a plurality of traffic (or voice) channels (TCHs) which operate in analog mode for analog systems and digital mode for digital systems. Sets of assigned channels are different for adjacent cells, and such sets are not repeated within the system except for cells that are far enough away from each other to minimize the likelihood of adjacent channel or co-channel interference.
The most widely recognized code division multiple access communications system known in the art is that system defined by TIA/EIA Interim Standard IS-95A. In a digital code division multiple access communications system, plural cells operate over the same frequency in a spread spectrum fashion. Digitized speech is multiplied by a pseudo-random code to "spread" the spectrum and provide the voice channels for subscriber communications. By pseudo-random it is meant that the multiplying data appears to be random but is in fact following a known direct sequence. The output signal is then mixed with the carrier frequency assigned to the cell and broadcast. To effectuate control over system operation, a control channel is also spread spectrum broadcast in each cell. For effectuating cell selection by the mobile stations, a pilot channel is also spread spectrum broadcast, with the cells distinguished from each other by the use of differing code phases for the pilot channel broadcasts.
As the system configuration and operation of both a conventional frequency division multiple access communications system and a conventional code division multiple access communications system are well known to those skilled in the art, detailed technical information need not be provided herein. However, technical information concerning these topics may be obtained by referring to a number of available documents. For example, for a description of the use of FDMA and TDMA/FDMA techniques in a multiple access communications system, reference is made to the text "Cellular Radio Systems", D. Balston, et al. (editors), Artech House, 1993. A number of known United States Patent provide a description of the use of CDMA techniques in multiple access communications systems.
In such multiple access communications systems, a handoff scheme of some sort is implemented to allow a call communication to continue in instances when a mobile station crosses the boundary between two cells. In one commonly used handoff scheme for analog systems, handoff is initiated when received signal strength in a given cell for a mobile station communication falls below a given threshold. The system then determines whether a neighboring cell can receive mobile station communications with a greater signal strength. If such a cell is identified, that cell is selected for the handoff. In another commonly used handoff scheme for digital systems, the mobile station additionally or alternatively makes downlink signal strength measurements with respect to the given cell and the control (pilot or reference) channels of its neighboring cells. These measurements are reported to the system for use in making the handoff determination and selecting the cell to which handoff occurs. In either of these schemes, the system informs the mobile station as to which cell is to be used in the future for continuing the call, and a corresponding re-routing of the call to that cell is made to coincide with a mobile station channel switch.
In conventional FDMA type cellular communications systems, each adjacent cell operates with a different set of radio frequencies. Accordingly, for each of the foregoing handoff schemes, the handoff must comprise a "hard" handoff. By this it is meant that the mobile station must change transceiver operation from a radio frequency used in and assigned to the current cell to a radio frequency used in and assigned to the new cell. Due to the limited functionalities of the transceiver for the mobile station, the mobile station must break its communication link with the currently serving cell before making the communication link with the newly serving cell.
In CDMA communications systems, however, the same frequency band can be used for many adjacent cells. Accordingly, for each of the foregoing handoff schemes, the handoff may comprise a "soft" handoff. By this it is meant that the mobile station need not necessarily change frequencies when switching communications from the current cell to the new cell. Communication with the mobile station is often simultaneously effectuated by two or more cells using the same frequency band, with the call switched between the two or more cells as measured signal strength dictates. When the signal strength measurements confirm a complete mobile station transition into the newly serving cell, communications utilizing the prior cell are discontinued.
As multiple access communications systems continue to be installed and upgraded, instances are sure to arise where one geographic area is provided service through a frequency division multiple access communications system, and an adjacent geographic area is provided service through a code division multiple access communications system. It may then become necessary to effectuate a handoff of a mobile station between the two systems. Such a handoff could only take place for mobile stations that are dual mode in nature such that they support both narrowband (FDMA) and wideband (CDMA) communications. As a practical matter, however, even assuming dual mode operation, certain technical limitations serve to preclude such a handoff. First, when a mobile station is communicating using the code division multiple access communications system, the base stations in the frequency division multiple access communications system cannot make signal strength measurements on the mobile station spread spectrum signal transmissions for the purpose of confirming handoff suitability. Furthermore, the frequency division multiple access communications system does not support the transmission of a pilot channel which a mobile station operating in spread spectrum mode must monitor for purposes of identifying target cells suitable for handoff.
Several solutions have been proposed to facilitate mobile station handoff between cells of a code division multiple access communications system and adjacent cells of a frequency division multiple access communications system. In a first solution, handoff is forced to a frequency division multiple access communications mode of operation when the mobile station moves within a ring of cells at the periphery of the code division multiple access communications system prior to handoff to the frequency division multiple access communications system. This solution is not favored because it wastes potential code division multiple access communications coverage area. In another solution, each frequency division multiple access communications system base station for cells positioned along the border with the code division multiple access communications system includes a wideband receiver for receiving spread spectrum mobile station communications for evaluation and processing by the frequency division multiple access communications system in making a handoff determination. Such a wideband receiver would necessarily have to be quite intelligent in order to receive and decode CDMA signals from mobile stations. This solution is accordingly not favored because of the added cost and complexity of supporting wideband receiver installation and operation.